Nodulation of legumes in inland valley soils of Ghana

Abstract

Because most African farmers cannot afford the purchase of chemical fertilizers to correct soil nutrient deficiencies, alternatives such as increased use of biological nitrogen fixation are most useful. For best results, sound recommendations are needed on the use of different legumes, their rhizobial requirement and management for optimum nitrogen fixation. In West Africa however, except for the highly popular legumes such as cowpea, soybean and groundnut, few reliable data are available on the abilities of other legumes to nodulate and fix nitrogen with naturally occurring strains of rhizobia in various soils and thus on the need for rhizobial inoculation. A screen-house study was conducted at the University of Ghana, Legon, to assess the potential of 13 commonly grown legumes in Ghana to form nodules with native rhizobia from eight inland valley soils and to identify those for which further research would be needed to enhance their nodulation. Two of the soils had indigenous rhizobia that induced formation of nodules on all the legumes evaluated. Rhizobia in the remaining soils induced the formation of nodules on more than 75% of the legumes studied. However, numbers of nodules plant-1 on most of the legumes were low and were influenced by (i) the type of soil, (ii) legume and (iii) the soil x legume interaction. Population densities of rhizobia capable of forming nodules with legumes in the studied soils estimated by the most probable number method (MPN) ranged from very low (negligible to 1.7 x 101 cells g-1 soil for those that could form nodules with mucuna) to high (up to 1.7 x 105 cells g-1 soil and 3.1 x 105 cells g-1 soil for cowpea and green gram, respectively). Nodule number per plant correlated significantly with some important indices of soil fertility; namely %N, available NO3 - and % organic C, with r values of 0.39*, 0.60* and 0.40*, respectively. Inoculation of mucuna increased nodulation by between 3 to 20-fold and N2 fixation by 18 to 98 percent depending on the soil type. Non-inoculated mucuna absorbed a high proportion of its N (up to 60%) from the soil and may mine the soil of native N if harvested for seed. The low rhizobial populations and poor nodulation are indications for the necessity for inoculating many of these legumes in similar soils in West Africa.